The invention relates to wireless communications systems in general, and more particularly to method and apparatus for reducing interferences in signal detection for the wireless communications systems.
A wireless communications system is designed to allow a large number of mobile terminals to access communications infrastructures via wireless media simultaneously. A popular cellular structure for the wireless communications system is to place multiple base stations to cover a large area having multiple cells, with each base station being an access point covering one cell. Once physically entered into a cell, a plurality of mobile terminals may select the same base station of the cell to conduct the communication at the same time.
Since the frequency spectrum is limited, in order to increase the use efficiency of the frequency spectrum, it is desirable for some cells (with their corresponding mobile terminals and base stations) to use the same radio frequency (RF). The so-called “frequency reuse” mechanism can be used along with various access techniques, such as Code Division Multiple Access (CDMA), Multi-carrier CDMA (MC-CDMA), and Time Division Multiple Access (TDMA). It is a challenge to maximize the frequency reuse, and various mechanisms have been implemented for assigning and arranging the frequencies.
In a wireless communications system implemented with the frequency reuse scheme, a signal transmitting in one cell is degraded by signals transmitting in its neighboring cells. Such degradation is referred to as “inter-cell interference.” Inter-cell interference and thermal noise are collectively referred to as “other interference” (OI).
The mobile terminal and the base station communicates through a wireless communications channel, and the wireless communications channel can be impaired by channel delay dispersion due to multi-path channels or RF chain distortion. Depending on the modulation types and access schemes, channel delay dispersion can cause various kinds of signal degradation. For example, inter-symbol interference (ISI) is often seen in the system using TDMA, while inter-code interference (ICI) and multiple access interference (MAI) are usually found in the system using CDMA or MC-CDMA.
For the base stations equipped with smart antennas, OI can be reduced using a nulling or beamforming algorithm for the downlink, uplink or both. ISI, MAI, and ICI can be reduced using methods, such as equalization, joint detection, nulling, or beamforming, for the uplink, downlink or both. The effectiveness for utilizing these interference reduction methods largely hinges on the accurate knowledge of channel characteristics available to both the transmitter and receiver of the system.
Specifically, the channel characteristics include the channel delay dispersion and OI characteristics. It is well known that a training sequence can be used to estimate channel characteristics. However, even using the training sequence, the accuracy of the estimation can be greatly impaired by the presence of inter-cell interference. Therefore, the methods for reducing interference, such as joint detection or equalization, may not work very well.
What is needed is a method and system to accurately estimate channel characteristics for improving the effectiveness of the techniques used for reducing interferences of a wireless communications system.